/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.atomic;

import java.util.function.UnaryOperator;
import java.util.function.BinaryOperator;
import sun.misc.Unsafe;

/**
 * An object reference that may be updated atomically. See the {@link
 * java.util.concurrent.atomic} package specification for description
 * of the properties of atomic variables.
 *
 * @param <V> The type of object referred to by this reference
 * @author Doug Lea
 * @since 1.5
 */
public class AtomicReference<V> implements java.io.Serializable {

  private static final long serialVersionUID = -1848883965231344442L;

  private static final Unsafe unsafe = Unsafe.getUnsafe();
  private static final long valueOffset;

  static {
    try {
      valueOffset = unsafe.objectFieldOffset
          (AtomicReference.class.getDeclaredField("value"));
    } catch (Exception ex) {
      throw new Error(ex);
    }
  }

  private volatile V value;

  /**
   * Creates a new AtomicReference with the given initial value.
   *
   * @param initialValue the initial value
   */
  public AtomicReference(V initialValue) {
    value = initialValue;
  }

  /**
   * Creates a new AtomicReference with null initial value.
   */
  public AtomicReference() {
  }

  /**
   * Gets the current value.
   *
   * @return the current value
   */
  public final V get() {
    return value;
  }

  /**
   * Sets to the given value.
   *
   * @param newValue the new value
   */
  public final void set(V newValue) {
    value = newValue;
  }

  /**
   * Eventually sets to the given value.
   *
   * @param newValue the new value
   * @since 1.6
   */
  public final void lazySet(V newValue) {
    unsafe.putOrderedObject(this, valueOffset, newValue);
  }

  /**
   * Atomically sets the value to the given updated value
   * if the current value {@code ==} the expected value.
   *
   * @param expect the expected value
   * @param update the new value
   * @return {@code true} if successful. False return indicates that the actual value was not equal
   * to the expected value.
   */
  public final boolean compareAndSet(V expect, V update) {
    return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
  }

  /**
   * Atomically sets the value to the given updated value
   * if the current value {@code ==} the expected value.
   *
   * <p><a href="package-summary.html#weakCompareAndSet">May fail
   * spuriously and does not provide ordering guarantees</a>, so is
   * only rarely an appropriate alternative to {@code compareAndSet}.
   *
   * @param expect the expected value
   * @param update the new value
   * @return {@code true} if successful
   */
  public final boolean weakCompareAndSet(V expect, V update) {
    return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
  }

  /**
   * Atomically sets to the given value and returns the old value.
   *
   * @param newValue the new value
   * @return the previous value
   */
  @SuppressWarnings("unchecked")
  public final V getAndSet(V newValue) {
    return (V) unsafe.getAndSetObject(this, valueOffset, newValue);
  }

  /**
   * Atomically updates the current value with the results of
   * applying the given function, returning the previous value. The
   * function should be side-effect-free, since it may be re-applied
   * when attempted updates fail due to contention among threads.
   *
   * @param updateFunction a side-effect-free function
   * @return the previous value
   * @since 1.8
   */
  public final V getAndUpdate(UnaryOperator<V> updateFunction) {
    V prev, next;
    do {
      prev = get();
      next = updateFunction.apply(prev);
    } while (!compareAndSet(prev, next));
    return prev;
  }

  /**
   * Atomically updates the current value with the results of
   * applying the given function, returning the updated value. The
   * function should be side-effect-free, since it may be re-applied
   * when attempted updates fail due to contention among threads.
   *
   * @param updateFunction a side-effect-free function
   * @return the updated value
   * @since 1.8
   */
  public final V updateAndGet(UnaryOperator<V> updateFunction) {
    V prev, next;
    do {
      prev = get();
      next = updateFunction.apply(prev);
    } while (!compareAndSet(prev, next));
    return next;
  }

  /**
   * Atomically updates the current value with the results of
   * applying the given function to the current and given values,
   * returning the previous value. The function should be
   * side-effect-free, since it may be re-applied when attempted
   * updates fail due to contention among threads.  The function
   * is applied with the current value as its first argument,
   * and the given update as the second argument.
   *
   * @param x the update value
   * @param accumulatorFunction a side-effect-free function of two arguments
   * @return the previous value
   * @since 1.8
   */
  public final V getAndAccumulate(V x,
      BinaryOperator<V> accumulatorFunction) {
    V prev, next;
    do {
      prev = get();
      next = accumulatorFunction.apply(prev, x);
    } while (!compareAndSet(prev, next));
    return prev;
  }

  /**
   * Atomically updates the current value with the results of
   * applying the given function to the current and given values,
   * returning the updated value. The function should be
   * side-effect-free, since it may be re-applied when attempted
   * updates fail due to contention among threads.  The function
   * is applied with the current value as its first argument,
   * and the given update as the second argument.
   *
   * @param x the update value
   * @param accumulatorFunction a side-effect-free function of two arguments
   * @return the updated value
   * @since 1.8
   */
  public final V accumulateAndGet(V x,
      BinaryOperator<V> accumulatorFunction) {
    V prev, next;
    do {
      prev = get();
      next = accumulatorFunction.apply(prev, x);
    } while (!compareAndSet(prev, next));
    return next;
  }

  /**
   * Returns the String representation of the current value.
   *
   * @return the String representation of the current value
   */
  public String toString() {
    return String.valueOf(get());
  }

}
